AIS and ARP target do not show same position

[GWB]

I have a question about this. The radar has locked up an ARP contact on the screen, but this contact is also transmitting AIS. The contact is displayed on the radar screen as two different contacts... not on top of each other as you would expect.
The range is 1.5 miles in the picture. Target 8 is also the AIS green icon

 

[chtucker]

My guess would be that the difference is that the AIS position is from a faster moving Class B AIS target. Class B updates its position every 30 seconds. 30 seconds at 30mph is a 1/4 mile. Miss that one transmission from a Class B 30mph target at it could be 1/2 mile difference. That is probably the worst case scenario, but you get the idea.

Class A AIS transits its position much more often when moving above 14 knots.

 

[GWB]

Thanks for that. That would make sense normally, but that target is a slow moving shrimp fishing boat...moving at about 3k
I have another question about how the radar determines target range/distance. There is GPS LL being input to the radar, but if the cursor is moved to be over the target, the LL the cursor reports is different from the LL on the target boat....up to 1/10 of a mile.
I have asked the skipper if his targets appear dead ahead when they are dead ahead as my first thought is that the heading alignment had not been done on setup... but he says a buoy dead ahead does appear dead ahead.
I'm at a loss.

 

[Johnny Electron]

If the radar video is perfectly aligned to the bow of the boat as you describe it sounds like your heading data being provide is off or not selected as the correct type (True/Mag). Put your radar to show magnetic heading and compare the heading numbers shown to your steering compass and the COG from the plotter. I think you will find they are not in agreement.

 

[GWB]

Thanks for the reply. I was under the impression that a GPS such as a Furuno GP32 calculates COG relative to true north. Thus the radar should be set on true north as well ... it is receiving HDT, HDG and HDM sentances.

 

[Johnny Electron]

Don't confuse GPS COG with real heading from your compass/pilot. To show AIS on a radar you need both correct GPS (for position) and correct Heading (not COG) to properly plot a target.

 

[GWB]

Right. My observation was in response to your reply about the COG, magnetic heading and steering compass.. The magnetic heading is not going to be the same as COG anyway, correct?

 

[Johnny Electron]

If the display is setup for magnetic both the GPS and the heading will be displayed in magnetic and should match when moving straight without cross currents and no recent turns.

 

[GWB]

Thanks